Electron Microscopy in the History of Soil Mineralogy.

Electron microscopy continues to be an important contributor to the progress of soil science. Transmission electron microscope development in 1938 and scanning electron microscope in 1952 helped lay the foundation for soil science as we know it today.  Soils are composed of particles of different sizes that require electron microscopes to study their size and numerous shapes, fabrics, and structural properties. 

Preparation technique may be as simple as drying a drop of a clay suspension on an organic film or mesh.  Modern TEM instruments have a digital camera and fast Fourier transform that improve instruction and research performance.

TEM is required to characterize the finest particles and SEM is favored for coarser particles i.e. silts and sands. Energy dispersive spectroscopy for chemical elemental analyses is available on TEM and SEM instruments and it is a frequent compliment to morphological observations by SEM.  

Electron diffraction data contributed to elucidating the structure of imogolite and differentiating it from allophane in volcanic soils. 

Unique particle morphology e.g. smectite blisters formed in vermiculite or spheroidal halloysite formed in pores of volcanic glass. Structural differences in todorokite cause multiple sizes of the unit cells shown by TEM. Small crystals of iron or manganese oxides at low concentrations are seen by TEM and identified by their lattice fringes.  Fibrous minerals such as palygorskite can be located when only traces are present, unlikely to be detected by XRD. Smectites differ in aflatoxin adsorption related to subtle differences in particle size and shape characterized by lattice fringes shown by TEM. 

Some soil minerals copy the morphology of the environment of formation e.g. opaline silica characterized by its unique morphology by SEM. The cellular structure of the charcoal in soils can be studied with SEM.